[skyline889]

I just received my toaster MPX3 today and it has a few problems. I'll probably try and contact Mikhail sometime this week, but I thought that maybe you guys could help me diagnose what's wrong with my amplifier in the mean time.

The main problem that's bothering me right now is a constant hum coming from the left channel. Even when the volume is turned completely down, I still get a constant hum. I tried switching tubes, I plugged the amplifier into it's own socket rather than the power strip I use for the rest of my components, I tried removing the interconnects, but no matter what, there is still a faint hum coming from the left channel. What could be causing it? Also when the volume is turned to the max, there's an even louder buzz coming from both channels.

The other problem I'm having, is that in transit, the rear power switch broke off. I can still turn the amplifier on and off with the part that's left but when it was plugged into the same power strip as my other components, it sent a pop through my speakers when the amplifier was turned on or off. Since I plugged the amp directly into the wall, I don't get the pop through my speakers anymore, but that still doesn't stop me from worrying that there maybe something wrong with the power section of the amplifier. Any help would be greatly appreciated.

 

[sacd lover]

Quote:

Originally Posted by skyline889 /img/forum/go_quote.gif I just received my toaster MPX3 today and it has a few problems. I'll probably try and contact Mikhail sometime this week, but I thought that maybe you guys could help me diagnose what's wrong with my amplifier in the mean time. The main problem that's bothering me right now is a constant hum coming from the left channel. Even when the volume is turned completely down, I still get a constant hum. I tried switching tubes, I plugged the amplifier into it's own socket rather than the power strip I use for the rest of my components, I tried removing the interconnects, but no matter what, there is still a faint hum coming from the left channel. What could be causing it? Also when the volume is turned to the max, there's an even louder buzz coming from both channels. The other problem I'm having, is that in transit, the rear power switch broke off. I can still turn the amplifier on and off with the part that's left but when it was plugged into the same power strip as my other components, it sent a pop through my speakers when the amplifier was turned on or off. Since I plugged the amp directly into the wall, I don't get the pop through my speakers anymore, but that still doesn't stop me from worrying that there maybe something wrong with the power section of the amplifier. Any help would be greatly appreciated.

If the amp took a hard enough hit to break off part of the power switch you likely have some damage. The shock could also have caused some of the wiring to come into contact with the case or be to close to the power transformer; hence the hum. The hum you describe sounds like power supply noise and the area of damage is near the power supply. You could dig around in the amp and move some wires around that are to close to the power transformer with a pencil etc... But, since you are new tubes and would not understand the internal p2p wiring I would advise you NOT do so. The amp has high internal voltages and big caps that could discharge and give you a nasty shock.

Finally, the hum your hear at max volume is normal for a tube amp. The sound is called tube rush which is the thermal noise of the tube itself being amplified. The real issue is getting the amp dead quiet with no signal playing at your normal listening volume. The amp will likely clip at around 12-1 o'clock with a normal 2 volt source .... and running a tube amp up to max volume until you get tube rush is not indicative of any problem. I hate to disappoint you, but I would send the amp in for repair.

 

[nikongod]

to agree with sacd: if you are not well versed in working on tube amps, send it to mikhail. i would not even crack it open if the words "four-hundred volts or more" scares you, it hangs out in there, and it sticks around after the amp is off and unplugged...

 

[003]

My MPX3 slam toaster had the exact same hum problem (only in one channel, at max volume) as you. I sent it in for mikhail to fix, and he did free of charge. I only had to pay for shipping and the volume pot upgrade.

However, the repair took 4-5 months.

 

[skyline889]

Thanks for the help guys. I'd love to send the amp back to Mikhail for repair however, I don't have 5 months to wait and I also can't afford the round trip shipping from Hawai'i all the way up to Singlepower.

I'm handy with a soldering iron and I popped open the bottom yesterday however without a schematic or something wrong to look for, I don't think I'll be able to fix it myself. I'll try look for some misplaced or knocked about wiring or solder joints but I guess if that fails, I'll have to take it in to a local audio repair/restoration shop. 003, did Mikhail happen to tell you what was wrong with the amplifier? Maybe I could try look for the same thing. Also, the faint hum from the left channel is there at both minimum and maximum volume. The buzz I was hearing at maximum volume I think was probably the tube rush that Earl was describing. I've been a solid state guy since I got into this hobby so I guess I'm just unaccustomed to the different noises that tube gear can make as opposed to solid state.

 

[skyline889]

I've been doing some research and found a site that said if the hum is constant regardless of where the amplifier controls are set, it is almost definitely a problem with the power caps. Is there any to test whether the caps are in good shape or not?

 

[takezo]

Quote:

Originally Posted by skyline889 /img/forum/go_quote.gif I've been doing some research and found a site that said if the hum is constant regardless of where the amplifier controls are set, it is almost definitely a problem with the power caps. Is there any to test whether the caps are in good shape or not?

the best and surest way of testing a capacitor, without proper equipments,
is to replace it and see if SQ changes...you will want to drain the power
caps with a cheap 25k or 56k resistor in parallel connection with the caps using
an insulated glove or pliers...the drain is instanteous and just requires a
touch...

from your descriptions, it could be the power caps, filter caps or even the
connections might be the problem. cold solder joints or loose joints can
cause this problem. another could be reflux induced by live wires being too
close to each other...

 

[n_maher]

Did you have the package insured? If so, I'd document the damage and file a claim before you went any further.

 

[robinje]

Maybe its a simple ground loop hum problem. That's what I had with my MPX3. I installed a 2-prong "cheater" plug on the AC cable and never had the problem again. This $0.39 fix worked like a charm. I suggest you give it a shot.

Jim

 

[003]

The MPX3 should not produce noise from a ground loop at any volume. But it wouldn't hurt to test. Mikhail did not tell me what was wrong. Now I see your hum is a bit different than mine, although only in one channel, my hum was only audible with the volume up very high.

 

[KZEE]

Last winter I bought a tubed preamp and a tubed speaker amp and I ran into every ground loop problem imaginable with those components. It turned out that I needed to have the tubed gear, including my source, isolated on it's own circuit. To check for a ground loop situation use cheater plugs on both the MPX and your source and plug them both into an outlet that has no other stereo or AV equipment plugged into it - if the MPX still hums then it more than likely is not a ground loop problem. Also make sure that your interconnects don't come within two feet or so of any AC power cables.

 

[vcoheda]

Quote:

Originally Posted by KZEE /img/forum/go_quote.gif Also make sure that your interconnects don't come within two feet or so of any AC power cables.

how can you do that?

 

[nikongod]

in a headphone based system, the opportunity for noise pickup from a power-cord is small. the power "flow" simply isnt there to make the EMF to leak out.

you should not need to put a cheater plug on ANYTHING in your system unless its plugged into different outlets with different ground references. in my system there are NO cheater plugs, BUT everything is plugged into a monster-power conditioner (glorified surge bar

)

if moving a couple wires around in the case has not gotten rid of the noise, i would send it to mikhail (and buy my MPX3 to hold you over...) totally serious for the first bit, medium for the second. although it will take forever and 2 days to get it back, he is a great guy to deal with.

incidentally, there are 2 wires that go from the tube closest to the outputs to the middle and then to the input tube (the heater wires) there is also a wire that goes from the input tube to the tube closest to the output on the same side. odds are good its too close to the heater supply, and picking up noise. move it around, see what happens.

 

[KZEE]

Quote:

Originally Posted by vcoheda /img/forum/go_quote.gif how can you do that?

How can you do that?

I assumed it wouldn't be necessary to state the obvious, but that's what I get for assuming.

Yes RCA or XLR jacks and power cord inlets on stereo components are usually located in close proximity to each other at the point at which they enter the component's chassis, and thus it obviously would be impossible to keep interconnects and power cords separated two feet from each other at the point that they enter the chassis. However in most systems power cords and interconnects at the midpoint of their runs have enough slack in them to be able to be very close to, or even touch, one another, and it's at that point that I'm suggesting keeping a two foot separation in order to minimize the possibility of AC hum contaminating the signal through the interconnects.

 

[KZEE]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif in a headphone based system, the opportunity for noise pickup from a power-cord is small. the power "flow" simply isnt there to make the EMF to leak out.

Because the speakers are so close to your ears headphone systems are in general much more sensitive to extraneous noise pickup than are speaker based systems, and as a result power-cord noise pickup in a tube-based headphone system can actually be more audible than with a speaker based system. And as many tube-based system owners - headphone or not - can tell you, a couple of very effective ways to reduce the chances of AC hum entering one's system is through power-cord/interconnect isolation, and through single power-cord plug-in points.

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif you should not need to put a cheater plug on ANYTHING in your system unless its plugged into different outlets with different ground references. in my system there are NO cheater plugs, BUT everything is plugged into a monster-power conditioner (glorified surge bar )

Perhaps I should have been clearer about it in my post, but the use of cheater plugs was suggested only as a DIAGNOSTIC TOOL to find out if there is or is not a ground loop problem. Having said that, I do however know some very experienced people in the audio world that use cheater plugs to deal with stubborn ground-loop problems that can't be eliminated any other way. I've used them myself on occasion when no other solution would work, and I would use them again if I had to. As always YMMV, and the choice is up to the individual audiophile.